Low-Thermal-Expansion Polycrystalline Zirconyl Phosphate Ceramic: Solid-Solution and Microcracking-Related Properties

The effects of the addition of Ta205 or Nb205 on the sintering and refractory properties of 2ZrO₂·P₂O₅-based ceramics have been investigated. The solid solution with Nb₂O₅ has a lower average thermal expansion coefficient than the pure phase. The strength of the polycrystalline ceramic is reduced when the grain size exceeds the transition grain size for microcracking. The thermal expansion of the ceramic is abruptly lowered when the grain size exceeds a value which should be termed the "second transition grain size" for additional wider microcracking. SiO₂ was found most effective for inhibiting grain growth to prevent microcracking and for obtaining a strong ceramic.     

Thermal Expansion Behavior of NaZr2(PO4)3Type Compounds

The thermal expansion of the skeletal framework was essentially zero for NaZr₂(PO₄)₃-type compounds; the interstitialion, e.g., Na⁺, was primarily responsible for the total thermal expansion. The composition dependence of the thermal expansion is discussed in terms of the amounts, crystallographic sites, and ionic radii of the interstitial ions. The mechanism which results in low thermal expansion was clarified, particularly for KZr₂(PO₄)₃, in which a larger ion is substituted for Na⁺, and NbZr(PO₄)₃, which does not contain Na⁺. Polycrystalline ceramics formed from these crystals might be useful as thermal-shock-resistant materials.     

Dielectric and Electrostrictive Properties of Ferroelectric Relaxors in the System Pb(Mg1/3Nb2/3)O3:PbTiO3: Ba(Zn1/3Nb2/3)O3

Ceramic dielectrics which have been fabricated in the Pb(Mg_1/3 Nb_2/3)O₃:PbTiO₃:Ba(Zn_1/3Nb_2/3)O₃ composition system are shown to exhibit two distinct dielectric maxima, both of which show the characteristic loss spectra of ferroelectrics with diffuse phase transitions. The height of the individual maxima can be controlled by the Zn:Mg ratio in the starting material and, in suitably chosen compositions, a wide range of almost temperature-independent high dielectric permittivity is possible. These dielectrics show strong electrostrictive deformations under high electric fields but the electrostrictive strain is much less temperature-sensitive than in other relaxors.     

Low-Thermal-Expansion Polycrystalline Zirconyl Phosphate Ceramic

The synthesis of thermal-shock-resistant material from zirconyl phosphate, 2ZrO₂P₂O₅, was investigated. Modifications of the compound were examined; only the a phase appears to be thermodynamically stable. Sintering of a-phase powder to a dense ceramic was promoted by adding a metal oxide. Crystalline zirconyl phosphate has a low average thermal expansion coefficient. The relation of microcracking to grain size and strength was studied using specimens sintered under varied conditions. A strong low-thermal-expansion polycrystalline ceramic was obtained by controlling grain growth.     

Focusing of high power millimeter-wave radiation by a quasi-optical antenna system

The focusing of millimeter-wave radiation from a high power 60 GHz gyrotron is studied using a two-dimensional ellipso-parabolic focusing antenna system. Fine focusing with an elliptic reflector gives a sheet beam of about 10 mm in thickness even at a high power input of about 100 kW. The addition of a parabolic reflector produces a finely focused and nearly Gaussian beam of about 10 mm in diameter with an energy density higher than lOOkWcm^-2. Spatial distributions of beam power are compared with calculations and good agreement is obtained.